[bugfix] Parse video metadata more accurately; allow Range in fileserver (#1342)

* don't serve unused fields for video attachments

* parse video bitrate + duration more accurately

* use ServeContent where appropriate to respect Range

* abstract temp file seeker into its own function

internal/api/model/content.go

@@ -21,6 +21,7 @@ package model
 import (
 	"io"
 	"net/url"
+	"time"
 )
 
 // Content wraps everything needed to serve a blob of content (some kind of media) through the API.
@@ -29,6 +30,8 @@ type Content struct {
 	ContentType string
 	// ContentLength in bytes
 	ContentLength int64
+	// Time when the content was last updated.
+	ContentUpdated time.Time
 	// Actual content
 	Content io.ReadCloser
 	// Resource URL to forward to if the file can be fetched from the storage directly (e.g signed S3 URL)